Display device and display module

ABSTRACT

To provide a display device or the like that enables stable curing of a resin. The display device includes a first circuit and a second circuit over the same substrate. The first circuit has a function of performing display; the second circuit has a function of driving the first circuit; the second circuit includes a transistor and a capacitor; the transistor includes an oxide semiconductor layer over a first insulating layer; the capacitor includes a first conductive layer, a second insulating layer, and a second conductive layer; the first conductive layer is positioned over the first insulating layer; one of a source and a drain of the transistor is electrically connected to the second conductive layer; and the first conductive layer and the oxide semiconductor layer include the same metal element.

BACKGROUND OF THE INVENTION I. Field of the Invention

One embodiment of the present invention relates to a display device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of the invention disclosed inthis specification and the like relates to an object, a method, or amanufacturing method. In addition, one embodiment of the presentinvention relates to a process, a machine, manufacture, or a compositionof matter. Specifically, examples of the technical field of oneembodiment of the present invention disclosed in this specificationinclude a semiconductor device, a display device, a light-emittingdevice, a power storage device, a memory device, a method for drivingany of them, and a method for manufacturing any of them.

2. Description of the Related Art

Display devices in which a display region (a pixel portion) and aperipheral circuit (a driver portion) are provided in the same substratehave been widely used. For example, Patent Document 1 discloses atechnique of using oxide semiconductor transistors in the display regionand the peripheral circuit.

REFERENCE Patent Document Patent Document 1 Japanese Published PatentApplication No. 2007-123861 SUMMARY OF THE INVENTION

For example, in the manufacture of a display device that uses a liquidcrystal element and includes a display region (a pixel portion) and aperipheral circuit (a driver portion) in the same substrate, thesubstrate is attached to another substrate with use of a resin. Theresin, which is cured by a variety of methods after being formed on thesubstrates, needs to be cured sufficiently for the attachment.

An object of one embodiment of the present invention is to provide adisplay device or the like that enables stable curing of a resin.

Another object of one embodiment of the present invention is to providea display device or the like that is inexpensive and has improvedproductivity.

Another object of one embodiment of the present invention is to providea display device or the like including a capacitor portion capable ofincreasing capacitance without increasing its area.

Another object of one embodiment of the present invention is to providea high-definition display device.

Another object of one embodiment of the present invention is to providea lightweight display device.

Another object of one embodiment of the present invention is to providea highly reliable display device.

Another object of one embodiment of the present invention is to providea low-power display device.

Another object of one embodiment of the present invention is to providea large-area display device.

Another object of one embodiment of the present invention is to providea novel display device or the like.

Note that the description of these objects does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention is a display device in which afirst circuit and a second circuit are provided over the same substrate.The first circuit has a function of performing display; the secondcircuit has a function of driving the first circuit; the second circuitincludes a transistor and a capacitor; the transistor includes an oxidesemiconductor layer over a first insulating layer; the capacitorincludes a first conductive layer, a second insulating layer, and asecond conductive layer; the first conductive layer is positioned overthe first insulating layer; one of a source and a drain of thetransistor is electrically connected to the second conductive layer; andthe first conductive layer and the oxide semiconductor layer include thesame metal element.

In the aforementioned display device, the first conductive layer and thesecond conductive layer can have a light-transmitting property.

In the aforementioned display device, the first conductive layer caninclude a region with a hydrogen concentration higher than that of theoxide semiconductor layer.

In the aforementioned display device, the second insulating layer caninclude a silicon nitride film.

In the aforementioned display device, the first circuit can include aliquid crystal element.

In the aforementioned display device, the first circuit can include anorganic EL element.

One embodiment of the present invention is a display device in which afirst circuit and a second circuit are provided over the same substrate.The first circuit has a function of performing display; the secondcircuit has a function of driving the first circuit; the second circuitincludes a transistor, a first capacitor, and a second capacitor; thetransistor includes an oxide semiconductor layer over a first insulatinglayer; the first capacitor includes a first conductive layer, the firstinsulating layer, and a second conductive layer; the second capacitorincludes the second conductive layer, a second insulating layer, and athird conductive layer; the second conductive layer is positioned overthe first insulating layer; one of a source and a drain of thetransistor is electrically connected to the second conductive layer; thefirst conductive layer and the third conductive layer are electricallyconnected to each other; and the first conductive layer and the oxidesemiconductor layer include the same metal element.

In the aforementioned display device, the first conductive layer, thesecond conductive layer, and the third conductive layer can have alight-transmitting property.

Note that other embodiments of the present invention will be shown belowin the description of Embodiments and the drawings.

One embodiment of the present invention can provide a display device orthe like that enables stable curing of a resin.

Another embodiment of the present invention can provide a display deviceor the like that is inexpensive and has improved productivity.

Another embodiment of the present invention can provide a display deviceor the like including a capacitor portion capable of increasingcapacitance without increasing its area.

Another embodiment of the present invention can provide ahigh-definition display device.

Another embodiment of the present invention can provide a lightweightdisplay device.

Another embodiment of the present invention can provide a highlyreliable display device.

Another embodiment of the present invention can provide a low-powerdisplay device.

Another embodiment of the present invention can provide a large-areadisplay device.

Another embodiment of the present invention can provide a novel displaydevice or the like.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the objects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1E are circuit diagrams and top views illustrating a displaydevice of one embodiment of the present invention:

FIGS. 2A and 2B are cross-sectional views illustrating the displaydevice of one embodiment of the present invention:

FIGS. 3A and 3B are cross-sectional views each illustrating a transistorof one embodiment of the present invention:

FIGS. 4A and 4B are cross-sectional views each illustrating a displaydevice of one embodiment of the present invention;

FIGS. 5A to 5D are top views illustrating modes of circuits of oneembodiment of the present invention;

FIGS. 6A to 6D are top views illustrating modes of circuits of oneembodiment of the present invention;

FIGS. 7A to 7C are top views and a cross-sectional view illustrating adisplay device of one embodiment of the present invention:

FIGS. 8A to 8D are top views and a cross-sectional view illustrating adisplay device of one embodiment of the present invention;

FIGS. 9A and 9B are cross-sectional views each illustrating a transistorof one embodiment of the present invention;

FIGS. 10A to 10C are a top view and cross-sectional views illustrating adisplay device of one embodiment of the present invention.

FIGS. 11A to 11C are a top view and circuit diagrams illustrating adisplay device of one embodiment of the present invention; and

FIGS. 12A to 12D illustrate electronic devices of one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to drawings. Notethat the present invention is not limited to the description below, andit is easily understood by those skilled in the art that various changesand modifications can be made without departing from the spirit andscope of the present invention. Accordingly, the present inventionshould not be interpreted as being limited to the content of theembodiments below. Note that in the structures of the inventiondescribed below, the same portions or portions having similar functionsare denoted by the same reference numerals in different drawings, andthe description of such portions is not repeated.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

In this specification and the like, a transistor is an element having atleast three terminals: a gate, a drain, and a source. The transistor hasa channel region between the drain (a drain terminal, a drain region, ora drain electrode) and the source (a source terminal, a source region,or a source electrode), and current can flow through the drain, thechannel region, and the source.

Since the source and the drain of the transistor change depending on thestructure, operating conditions, and the like of the transistor, it isdifficult to define which is a source or a drain. Thus, it is possiblethat a portion functioning as the source and a portion functioning asthe drain are not called a source and a drain, and that one of thesource and the drain is referred to as a first electrode and the otheris referred to as a second electrode.

Note that in this specification, ordinal numbers such as first, second,and third are used to avoid confusion among components, and thus do notlimit the number of the components.

Note that in this specification, the expression “A and B are connected”means the case where A and B are electrically connected to each other inaddition to the case where A and B are directly connected to each other.Here, the expression “A and B are electrically connected” means the casewhere electric signals can be transmitted and received between A and Bwhen an object having any electric action exists between A and B.

Note that for example, any of the following expressions can be used forthe case where a source (or a first terminal or the like) of atransistor is electrically connected to X through Z1 (or not through Z1)and a drain (or a second terminal or the like) of the transistor iselectrically connected to Y through Z2 (or not through Z2), or the casewhere a source (or a first terminal or the like) of a transistor isdirectly connected to one part of Z1 and another part of Z1 is directlyconnected to X while a drain (or a second terminal or the like) of thetransistor is directly connected to one part of Z2 and another part ofZ2 is directly connected to Y.

Examples of the expressions include “X, Y, and a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor are electrically connected to each other such that X, thesource (or the first terminal or the like) of the transistor, the drain(or the second terminal or the like) of the transistor, and Y areelectrically connected to each other in this order”; “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”; and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder.” When the connection order in a circuit configuration is definedby an expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope.

Another example of the expression is “a source (or a first terminal orthe like) of a transistor is electrically connected to X through atleast a first connection path, the first connection path does notinclude a second connection path, the second connection path is a pathbetween the source (or the first terminal or the like) of the transistorand a drain (or a second terminal or the like) of the transistor, Z1 ison the first connection path, the drain (or the second terminal or thelike) of the transistor is electrically connected to Y through at leasta third connection path, the third connection path does not include thesecond connection path, and Z2 is on the third connection path.” It isalso possible to use the expression “a source (or a first terminal orthe like) of a transistor is electrically connected to X through Z1 onat least a first connection path, the first connection path does notinclude a second connection path, the second connection path includes aconnection path through the transistor, a drain (or a second terminal orthe like) of the transistor is electrically connected to Y through Z2 onat least a third connection path, and the third connection path does notinclude the second connection path.” It is also possible to use theexpression “a source (or a first terminal or the like) of a transistoris electrically connected to X through Z1 on at least a first electricalpath, the first electrical path does not include a second electricalpath, the second electrical path is an electrical path from the source(or the first terminal or the like) of the transistor to a drain (or asecond terminal or the like) of the transistor, the drain (or the secondterminal or the like) of the transistor is electrically connected to Ythrough Z2 on at least a third electrical path, the third electricalpath does not include a fourth electrical path, and the fourthelectrical path is an electrical path from the drain (or the secondterminal or the like) of the transistor to the source (or the firstterminal or the like) of the transistor.” When the connection path in acircuit configuration is defined by an expression similar to the aboveexamples, a source (or a first terminal or the like) and a drain (or asecond terminal or the like) of a transistor can be distinguished fromeach other to specify the technical scope.

Note that these expressions are only examples, and there is nolimitation on the expressions. Here, X, Y, Z1, and Z2 each denote anobject (e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer).

Note that in this specification, terms for explaining arrangement, suchas “over” and “under”, are used for convenience to describe thepositional relationship between components with reference to drawings.The positional relationship between components is changed as appropriatein accordance with a direction in which each component is described.Thus, the positional relationship is not limited to that described witha term used in this specification and can be explained with another termas appropriate depending on the situation.

In this specification, the term “parallel” indicates that the angleformed between two straight lines ranges from 10° to 10°, andaccordingly also includes the case where the angle ranges from 5° to 5°.The term “substantially parallel” indicates that the angle formedbetween two straight lines ranges from 30° to 30°. The term“perpendicular” indicates that the angle formed between two straightlines ranges from 80° to 100°, and accordingly also includes the casewhere the angle ranges from 85° to 95°. The term “substantiallyperpendicular” indicates that the angle formed between two straightlines ranges from 60° to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

Embodiment 1

In this embodiment, a structure example of a display device will bedescribed.

(Circuit Diagrams and Top Views of FIGS. 1A to 1E)

FIG. 1A illustrates a display device 10. In this specification and thelike, the display device 10 refers to, for example, a device including adisplay element such as a liquid crystal element. The display device 10includes a circuit 20 and a circuit 30 which are formed over the samesubstrate. The circuit 20 can mainly constitute a display regionincluding pixels. The circuit 30 can, for example, drive and control thecircuit 20. Alternatively, the circuit 30 may have a function as aprotective circuit or a test circuit. The circuit 30 is, for example, atleast one of a gate driver, a protective circuit, a precharge circuit, atest circuit, and the like. The circuit 30 may be provided on either orboth sides of the circuit 20. In addition, the display device 10 mayinclude a circuit 40 over the same substrate. Similarly to the circuit30, the circuit 40 can drive and control the circuit 20. The circuit 40is, for example, at least one of a source driver, a switch, and thelike. Furthermore, the display device 10 includes at least one of acontrol circuit, a power source circuit, a signal generation circuit, anoptical sheet, a touch sensor, a touch sensor driver circuit, aphotosensor, a backlight, a frame, and the like, and is also referred toas a display panel or a display module. In the display device 10, thecircuit 40 is not necessarily provided over the same substrate.

FIG. 1B is a basic circuit diagram of a pixel circuit included in thecircuit 20. The circuit illustrated in FIG. 1B includes a transistor, acapacitor, and a liquid crystal element. FIG. 1D is a top view of thecircuit 20. FIG. 2A is a cross-sectional view along a dashed-dotted lineA-A′ in the top view of the circuit 20.

FIG. 1C is a circuit diagram illustrating part of the structure of thecircuit 30. FIG. 1E is a top view of the circuit 30. FIG. 2B is across-sectional view along a dashed-dotted line B-B′ in the top view ofthe circuit 30. Note that the circuit illustrated in FIG. 1C can serveas, for example, part of a buffer circuit, and includes a transistor 11,a transistor 12, a transistor 13, and a capacitor C1. Although thecapacitor C1 is connected between a gate and a source or the transistor12 in FIG. 1C, one embodiment of the present invention is not limitedthereto and the capacitor C1 may be connected at another node. Also inFIG. 1C, signals and potentials such as VDD, VSS, and CLK are suppliedas an example; however, one embodiment of the present invention is notlimited thereto and another signal or potential may be supplied.

<Cross-Sectional Schematic View of Display Device>

Cross-sectional views of the display device 10 will be described withreference to FIGS. 2A and 21. Note that the description is made on theassumption that the display device 10 is mainly a display panel using aliquid crystal. However, the display element is not limited to a liquidcrystal element using a liquid crystal and may be another displayelement such as an organic EL element.

The display device is constituted by components provided between asubstrate 100 and a substrate 300. A liquid crystal element 330 isincluded between the substrate 100 and the substrate 300 (see FIG. 2A).

A liquid crystal layer 200 is scaled by an adhesive layer 400 providedbetween the substrate 100 and the substrate 300. At least part of theadhesive layer 400 can be provided over at least part of the circuit 30or over at least part of elements included in the circuit 30 asillustrated in FIG. 1A and FIG. 2B. Note that the adhesive layer 400 andat least part of the circuit 30 do not necessarily overlap with eachother. The adhesive layer 400 may be provided in the vicinity of atleast part of the circuit.

<<Circuit 20>>

First, a cross-sectional structure of a pixel portion included in thecircuit 20 will be described with reference to FIG. 2A.

A transistor 60 and a capacitor 70 are provided over the substrate 100.

<<Transistor 60>>

The transistor 60 includes an insulating layer 110, a conductive layer120, an insulating layer 130, a semiconductor layer 140, a conductivelayer 150, a conductive layer 160, an insulating layer 170, and aninsulating layer 180. The conductive layer 120 has a function as a gateelectrode. The insulating layer 130 has a function as a gate insulatingfilm. The semiconductor layer 140 has a function as a semiconductorlayer including a channel formation region. The conductive layer 150 hasa function as one of a source electrode and a drain electrode. Theconductive layer 160 has a function as the other of the source electrodeand the drain electrode. The conductive layer 160 is connected to aconductive layer 190. The insulating layer 170 can be used to protect achannel portion. The insulating layer 180 can be used to preventdiffusion of impurities.

In FIG. 2A, the area of a top surface of the semiconductor layer 140 ispreferably equal to or smaller than that of a top surface of theconductive layer 120.

In FIG. 2A, the conductive layer 120, the insulating layer 130, thesemiconductor layer 140, the conductive layer 150, or the conductivelayer 160 is a single layer; however, they may be a stack of two or morelayers. The layers in the stack may be formed using different materialsor the same material.

The transistor 60 illustrated in FIG. 2A is, but is not limited to, abottom-gate transistor. FIGS. 3A and 3B show modification examples ofthe transistor 60. Although the transistor 60 illustrated in FIG. 2A isa channel-etched transistor, it may be a channel-protective transistorincluding an insulating layer 165 as illustrated in the cross-sectionalview of FIG. 3A or may be a top-gate transistor as illustrated in thecross-sectional view of FIG. 3B.

When a transistor with an extremely low of T-state leakage current isused as the transistor 60 connected to a display element (e.g., theliquid crystal element 330), the time for holding image signals can beextended. For example, images can be held even when the frequency ofwriting image signals is higher than or equal to 11.6 μHz (once a day)and less than 0.1 Hz (0.1 times a second), preferably higher than orequal to 0.28 mHz (once an hour) and less than 1 Hz (once a second). Asa result, the frequency of writing image signals can be reduced, leadingto a reduction in the power consumption of the display device 10.Needless to say, the frequency of writing image signals can be higherthan or equal to 1 Hz, preferably higher than or equal to 30 Hz (30times a second). Further preferably higher than or equal to 60 Hz (60times a second) and less than 960 Hz (960 times a second).

An example of the transistor with an extremely low off-state leakagecurrent is a transistor in which an oxide semiconductor is used for asemiconductor layer. Specifically, for the semiconductor layer, an oxidesemiconductor containing at least indium (In), zinc (Zn), and M (M is ametal such as Al, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), which isrepresented by an In-M-Zn oxide, can be preferably used.

In the case where the voltage between a source and a drain is set toabout 0.1 V, 5 V, or 10 V, for example, the off-state currentstandardized on the channel width of the transistor in which an oxidesemiconductor is used for the semiconductor layer can be as low asseveral yoctoamperes per micrometer to several zeptoamperes permicrometer.

<<Oxide Semiconductor>>

As an oxide semiconductor used for the aforementioned semiconductorlayer 140, any of the following can be used, for example: anIn—Ga—Zn-based oxide, an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide,an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In-Cc-Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-basedoxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, anIn—Hf—Al—Zn-based oxide, and an In—Ga-based oxide.

Note that here, an “In—Ga—Zn-based oxide” means an oxide containing In,Ga, and Zn as its main components and there is no limitation on theratio of In:Ga:Zn. The In—Ga—Zn-based oxide may contain another metalelement in addition to In, Ga, and Zn.

From the above reason, the use of a transistor including an oxidesemiconductor allows fabrication of a low-power display device.

<<Capacitor 70>>

The capacitor 70 includes the conductive layer 190, the insulating layer180, and a conductive layer 210. The conductive layer 190 has a functionas one electrode of the capacitor 70. The conductive layer 210 has afunction as the other electrode of the capacitor 70. The insulatinglayer 180 is provided between the conductive layer 190 and theconductive layer 210. The conductive layer 190 is connected to thetransistor 60.

The conductive layer 210 as well as the semiconductor layer 140 isformed over the insulating layer 130.

When the transistor 60 includes an oxide semiconductor in thesemiconductor layer 140, the conductive layer 210 can be formed of thesame material as the semiconductor layer 140 over the insulating layer130. In that case, the conductive layer 210 is formed by processing afilm formed at the same time as the semiconductor layer 140, andtherefore contains elements similar to those in the semiconductor layer140. The conductive layer 210 has a crystal structure similar to ordifferent from that of the semiconductor layer 140. When the film formedat the same time as the semiconductor layer 140 includes impurities oroxygen vacancies, the film can have conductivity to be the conductivelayer 210. Typical examples of the impurities contained in theconductive layer 210 are a rare gas, hydrogen, boron, nitrogen,fluorine, aluminum, and phosphorus. Typical examples of the rare gasinclude helium, neon, argon, krypton, and xenon. Note that theconductive layer 210 has conductivity as an example; however, oneembodiment of the present invention is not limited to this example andthe conductive layer 210 does not necessarily have conductivitydepending on the case or circumstances. In other words, the conductivelayer 210 may have properties similar to those of the semiconductorlayer 140.

Although the semiconductor layer 140 and the conductive layer 210 areformed over the insulating layer 130 in the above manner, they havedifferent impurity concentrations. Specifically, the impurityconcentration of the conductive layer 210 is higher than that of thesemiconductor layer 140. For example, in the semiconductor layer 140,the hydrogen concentration measured by secondary ion mass spectrometryis lower than or equal to 5×10¹⁹ atoms/cm³, preferably lower than orequal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to1×10¹⁸ atoms/cm³, still further preferably lower than or equal to 5×10¹⁷atoms/cm³, and yet still further preferably lower than or equal to1×10¹⁶ atoms/cm³. In contrast, the hydrogen concentration in theconductive layer 210 measured by secondary ion mass spectrometry ishigher than or equal to 8×10¹⁹ atoms/cm³, preferably higher than orequal to 1×10²⁰ atoms/cm³, and further preferably higher than or equalto 5×10²⁰ atoms/cm³. In addition, the hydrogen concentration in theconductive layer 210 is greater than or equal to 2 times or greater thanor equal to 10 times that in the semiconductor layer 140.

When the hydrogen concentration in the semiconductor layer 140 is set inthe aforementioned range, generation of electrons serving as carriers inthe semiconductor layer 140 can be suppressed.

When an oxide semiconductor film formed at the same time as thesemiconductor layer 140 is exposed to plasma, the oxide semiconductorfilm is damaged and oxygen vacancies can be generated. For example, whena film is formed over the oxide semiconductor film by a plasma CVDmethod or a sputtering method, the oxide semiconductor film is exposedto plasma and oxygen vacancies are generated. Alternatively, when theoxide semiconductor film is exposed to plasma in etching treatment forformation of an opening in the insulating layer 170, oxygen vacanciesare generated. Alternatively, when the oxide semiconductor film isexposed to plasma of a mixed gas of oxygen and hydrogen, hydrogen, arare gas, ammonia, and the like, oxygen vacancies are generated.Alternatively, when impurities are added to the oxide semiconductorfilm, oxygen vacancies can be formed while the impurities are added tothe oxide semiconductor film. The impurities can be added by an iondoping method, an ion implantation method, a plasma treatment method,and the like. In the plasma treatment method, plasma is generated in agas atmosphere containing the impurities to be added, and ions of theimpurities accelerated by plasma treatment are made to collide with theoxide semiconductor film, whereby oxygen vacancies can be formed in theoxide semiconductor film.

When an impurity, e.g., hydrogen is contained in the oxide semiconductorfilm in which oxygen vacancies are generated by addition of impurityelements, hydrogen enters an oxygen vacant site and forms a donor levelin the vicinity of the conduction band. As a result, the oxidesemiconductor film has increased conductivity to be a conductor. Anoxide semiconductor film that has become a conductor can be referred toas an oxide conductor film. That is, it can be said that thesemiconductor layer 140 is formed of an oxide semiconductor and theconductive layer 210 is formed of an oxide conductor film. It can alsobe said that the conductive layer 210 is formed of an oxidesemiconductor film having high conductivity or a metal oxide film havinghigh conductivity.

Note that the insulating layer 180 preferably contains hydrogen. Sincethe conductive layer 210 is in contact with the insulating layer 180,hydrogen contained in the insulating layer 180 can be diffused into theoxide semiconductor film formed at the same time as the semiconductorlayer 140. As a result, impurities can be added to the oxidesemiconductor film formed at the same time as the semiconductor layer140.

Furthermore, the insulating layer 170 is preferably formed using anoxide insulating film containing more oxygen than that in thestoichiometric composition, and the insulating layer 180 is preferablyformed using an insulating film containing hydrogen. When oxygencontained in the insulating layer 170 is transferred to thesemiconductor layer 140 of the transistor 60, the amount of oxygenvacancies in the semiconductor layer 140 can be reduced and a change inthe electrical characteristics of the transistor 60 can be reduced. Inaddition, hydrogen contained in the insulating layer 180 is transferredto the conductive layer 210 to increase the conductivity of theconductive layer 210.

In the above manner, the conductive layer 210 can be formed at the sametime as the semiconductor layer 140 and is given conductivity alter theformation. Such a structure results in a reduction in manufacturingcosts.

Oxide semiconductor films generally have a visible light transmittingproperty because of their large energy gap. In contrast, an oxideconductor film is an oxide semiconductor film having a donor level inthe vicinity of the conduction band. Thus, the influence of lightabsorption due to the donor level is small, so that an oxide conductorfilm has a visible light transmitting property comparable to that of anoxide semiconductor film.

From the above reasons, the conductive layer 190 and the conductivelayer 210 have a light-transmitting property; as a result, the wholecapacitor 70 can have a light-transmitting property.

The aforementioned structure leads to an increase in the aperture ratioof a pixel in the display region. The increase in aperture ratio allowsdisplay to be performed at the same luminance with a weaker light from abacklight, reducing power consumption.

The conductive layer 190 has a function as a pixel electrode of theliquid crystal layer 200. The conductive layer 190 is formed using aconductive film that transmits visible light. For example, a materialincluding one of indium (In), zinc (Zn), and tin (Sn) can be used forthe conductive film that transmits visible light. Alternatively, theconductive layer 210 can be used as the pixel electrode. The conductivelayer 190 is formed using a conductive film that transmits visible lightor a conductive film that reflects visible light. When the conductivelayer 190 is formed using a conductive film that transmits visiblelight, a transmissive display device can be manufactured. When theconductive layer 190 is formed using a conductive film that reflectsvisible light, a reflective or semi-transmissive display device can bemanufactured.

Note that the conductive layer 160 may be connected to the conductivelayer 210 as illustrated in the cross-sectional view of FIG. 4A.

<<Liquid Crystal Element>>

The liquid crystal layer 200 is interposed between the conductive layer190 and a conductive layer 310 provided on the substrate 300 andreceives the electric field from the conductive layers 190 and 310, sothat the liquid crystal element 330 functions. Note that the liquidcrystal element 330 does not necessarily include the conductive layer310.

Examples of a driving method of the display device include a TN mode, anSTN mode, a VA mode, an axially symmetric aligned micro-cell (ASM) mode,an optically compensated birefringence (OCB) mode, a ferroelectricliquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC)mode, an MVA mode, a patterned vertical alignment (PVA) mode, an IPSmode, an FFS mode, and a transverse bend alignment (TBA) mode. Otherexamples of the driving method of the display device include anelectrically controlled birefringence (FCB) mode, a polymer dispersedliquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC)mode, and a guest-host mode. Note that one embodiment of the presentinvention is not limited to the above, and various liquid crystalelements and driving methods can be employed.

The liquid crystal element 330 may be formed using a liquid crystalcomposition including a liquid crystal exhibiting a nematic phase and achiral material. In that case, a cholesteric phase or a blue phase isexhibited. The liquid crystal exhibiting a blue phase has a shortresponse time of 1 msec or less. Since the liquid crystal exhibiting ablue phase is optically isotropic, alignment treatment is not necessaryand viewing angle dependence is small.

The substrate 300 includes the conductive layer 310 and the liquidcrystal layer 200 is interposed between the conductive layers 310 and190, so that the liquid crystal element 330 can be obtained. Thealignment of liquid crystal molecules included in the liquid crystallayer 200 can be controlled by the electric field between the conductivelayers 190 and 310.

<<Circuit 30>>

Next, the circuit 30 in the display device will be described.

The circuit 30 includes a transistor 1060 and a capacitor 1070.

<<Transistor 1060>>

The transistor 1060 includes an insulating layer 1110, a conductivelayer 1120, an insulating layer 1130, a semiconductor layer 1140, aconductive layer 1150, a conductive layer 1160, an insulating layer1170, and an insulating layer 1180. The conductive layer 1120 has afunction as a gate electrode. The insulating layer 1130 has a functionas a gate insulating film. The semiconductor layer 1140 has a functionas a semiconductor layer including a channel formation region. Theconductive layer 1150 has a function as one of a source electrode and adrain electrode. The conductive layer 1160 has a function as the otherof the source electrode and the drain electrode. The conductive layer1160 is connected to a conductive layer 1190. The insulating layer 1170can be used to protect a channel portion. The insulating layer 1180 canbe used to prevent diffusion of impurities.

<<Capacitor 1070>>

The capacitor 1070 includes the conductive layer 1190, the insulatinglayer 1180, and a conductive layer 1210. The conductive layer 1190 has afunction as one electrode of the capacitor 1070. The conductive layer1210 has a function as the other electrode of the capacitor 1070. Theinsulating layer 1180 is provided between the conductive layer 1190 andthe conductive layer 1210. The conductive layer 1190 is connected to thetransistor 1060.

Note that the conductive layer 1160 may be connected to the conductivelayer 1210 as illustrated in the cross-sectional view of FIG. 4B.

The transistor 1060 can be formed in the same process as the transistor60, and the components in the transistor 1060 can be formed using thesame material as those in the transistor 60. For example, the insulatinglayer 110 and the insulating layer 1110 can be formed in the sameprocess.

The capacitor 1070 can be formed in the same process as the capacitor70, and the components in the capacitor 1070 can be formed using thesame material as those in the capacitor 70. For example, the conductivelayer 210 and the conductive layer 1210 can be formed in the sameprocess. Alternatively, the conductive layer 1210 and the semiconductorlayer 1140 can be formed using the same material. In that case, theconductive layer 1210 is formed by processing a film formed at the sametime as the semiconductor layer 1140, and therefore contains elementssimilar to those in the semiconductor layer 1140. When the film formedat the same time as the semiconductor layer 1140 includes impurities oroxygen vacancies, the film can have conductivity to be the conductivelayer 1210. Note that the conductive layer 1210 has conductivity as anexample; however, one embodiment of the present invention is not limitedto this example and the conductive layer 1210 does not necessarily haveconductivity depending on the case or circumstances. In other words, theconductive layer 1210 may have properties similar to those of thesemiconductor layer 1140.

The conductive layer 1190 and the conductive layer 1210 have alight-transmitting property, for example; as a result, the wholecapacitor 1070 can have a light-transmitting property.

<<Adhesive Layer 400>>

The adhesive layer 400 has a function of attaching the substrates 100and 300.

An inorganic material, an organic material, a composite material of aninorganic material and an organic material, or the like can be used forthe adhesive layer 400.

For example, an organic material such as a light curable adhesive, areactive curable adhesive, a thermosetting adhesive, and/or an anaerobicadhesive can be used for the adhesive layer 400. Note that each of theadhesives can be used alone or in combination.

The light curable adhesive refers to, for example, an adhesive that iscured by ultraviolet rays, an electron beam, visible light, infraredlight, or the like.

Specifically, an adhesive containing an epoxy resin, an acrylic resin, asilicone resin, a phenol resin, a polyimide resin, an imide resin, apolyvinyl chloride (PVC) resin, a polyvinyl butyral (PVB) resin, anethylene vinyl acetate (EVA) resin, silica, or the like can be used forthe adhesive layer 400.

Particularly when a light curable adhesive is used, the material iscured rapidly, leading to shortening of the process time. In addition,involuntary curing of the adhesive due to environment can be preventedbecause curing starts with light irradiation. Furthermore, curing can beperformed at low temperatures to facilitate the control of processenvironment. From the above reasons, the use of a light curable adhesiveshortens the process time and reduces processing costs.

Note that in the case where a light curable adhesive is used for theadhesive layer 400 in FIG. 2B, the use of a light-absorbing orlight-reflecting material for a region in contact with the adhesivelayer 400 might cause insufficient adhesion because the adhesive layer400 cannot be irradiated with a sufficient amount of light. In oneembodiment of the present invention, however, a light-transmittingmaterial is used for both of the conductive layers 1190 and 1210 of thecapacitor 1070 in FIG. 2B, and thus, light passes through the capacitor1070. As a result, the adhesive layer 400 can be cured sufficiently andthe display device can be manufactured at low costs and with highproductivity. Moreover, the aforementioned structure enables a narrowerbezel (a reduction in the distance between a substrate edge and adisplay region). Note that in that case, at least part of the capacitor1070 does not necessarily overlap with the adhesive layer 400. Even whenthe capacitor 1070 does not overlap with the adhesive layer 400, lightcan be emitted to the adhesive layer 400 through the capacitor 1070 aslong as the capacitor 1070 is in the vicinity of the adhesive layer 400.

Although an example of using a transistor including an oxidesemiconductor is shown in this embodiment, one embodiment of the presentinvention is not limited to this example. Depending on the case orcircumstances, a transistor including a semiconductor material that isnot an oxide semiconductor may be used in one embodiment of the presentinvention.

For example, a transistor in which a Group 14 element, a compoundsemiconductor, an oxide semiconductor, or the like is used for thesemiconductor layer can be used. Specifically, a semiconductorcontaining silicon, a semiconductor containing gallium arsenide, anorganic semiconductor, a semiconductor containing silicon carbide, asemiconductor containing germanium, a semiconductor containing silicongermanium, a carbon nanotube, or the like can be used.

For example, single crystal silicon, polysilicon, or amorphous siliconcan be used for the semiconductor layer of the transistor.

Note that in this embodiment, the conductive layer 210 and thesemiconductor layer 140 are formed using the same material; however, oneembodiment of the present invention is not limited to this case. Forexample, in one embodiment of the present invention, the conductivelayer 210 and the semiconductor layer 140 may include differentmaterials depending on the case or circumstances. The same applies tothe conductive layer 1210 and the semiconductor layer 1140.

Note that in this embodiment, the conductive layer 1210 and thesemiconductor layer 1140 are formed using the same material; however,one embodiment of the present invention is not limited to this case. Forexample, in one embodiment of the present invention, the conductivelayer 1210 and the semiconductor layer 1140 may include differentmaterials depending on the case or circumstances.

Note that in this embodiment, the capacitor 1070 and the capacitor 70are formed in the same process and the components in the capacitor 1070are formed using the same material as those in the capacitor 70;however, one embodiment of the present invention is not limited to thisexample. For example, in one embodiment of the present invention, atleast part of the capacitors 1070 and 70 may be formed in differentprocesses depending on the case or circumstances. Also depending on thecase or circumstances, for example, at least part of the capacitors 1070and 70 may contain different materials in one embodiment of the presentinvention.

Note that the structures, methods, and the like described in thisembodiment can be used in appropriate combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, a modification example of the display device shownin Embodiment 1 will be described.

FIGS. 5A to 5D and FIGS. 6A to 6D show top views of the circuit 30.

In the capacitor 1070 in the circuit 30, the conductive layer 1190having a function as an electrode may have a variety of shapes asillustrated in FIGS. 5A to 5D in order to improve transmittance.

Alternatively, in the capacitor 1070 in the circuit 30, the conductivelayer 1210 having a function as an electrode may have a variety ofshapes as illustrated in FIGS. 6A and 6B in order to improvetransmittance. For example an opening 1270 may have a square shape or acircular shape.

Alternatively, in the circuit 30, not only the shape of the capacitorbut also the shapes of the conductive layers 1120 and 1150 may bechanged as illustrated in FIGS. 6C and 6D in order to improvetransmittance. For example, the conductive layer 1120, the conductivelayer 1150, or the like can be provided with an opening 1410, or theconductive layer 1120, the conductive layer 1150, or the like can beprovided with an opening 1420 to be connected with the conductive layer1190. When the openings 1270, 1410, and 1420 are thus provided tooverlap with the adhesive layer 400 or in the vicinity of the adhesivelayer 400, the adhesive layer 400 can be irradiated with light moreeasily.

In the aforementioned structure, the electrode of the capacitor 1070 inthe circuit 30 may be formed using a light-transmitting material or ametal material.

With the above shape, the light curable adhesive in the circuit 30 canbe cured more surely and the display device can be manufactured at lowcosts and with high productivity.

Note that the structures, methods, and the like described in thisembodiment can be used in appropriate combination with any of thestructures, methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, another mode of the display device 10, which is oneembodiment of the present invention, will be described with reference toFIGS. 7A to 7C.

FIG. 7A is a top view of the display device 10, FIG. 7B is a top view ofthe circuit 30, and FIG. 7C is a cross-sectional view along adashed-dotted line B-B′ in the top view of the circuit 30.

Note that the display device 10 illustrated in FIGS. 7A to 7C has thesame structure as the display device 10 illustrated in FIGS. 1A to 1Fand FIGS. 2A and 21 except the structure of the capacitor. Thus, thedescription of the transistor and the like is omitted.

<<Capacitor 1070>>

The capacitor 1070 includes the conductive layer 1190, the insulatinglayer 1180, and the conductive layer 1210. The conductive layer 1190 hasa function as one electrode of the capacitor 1070. The conductive layer1210 has a function as the other electrode of the capacitor 1070. Theinsulating layer 1180 is provided between the conductive layer 1190 andthe conductive layer 1210. The conductive layer 1190 is connected to thetransistor 1060.

<<Capacitor 1080>>

A capacitor 1080 includes a conductive layer 1220, the insulating layer1130, and the conductive layer 1210. The conductive layer 1220 has afunction as one electrode of the capacitor 1080. The conductive layer1210 has a function as the other electrode of the capacitor 1080. Theinsulating layer 1130 is provided between the conductive layer 1220 andthe conductive layer 1210.

<<Relationship Between the Capacitors 1070 and 1080>>

The capacitors 1070 and 1080 can be provided to overlap with each other.The conductive layer 1210 has a function as the electrodes of thecapacitors 1070 and 1080. The conductive layers 1190 and 1220 areelectrically connected to each other and can have the same potential.

The conductive layer 1210 is formed at the same time as thesemiconductor layer 1140 and is given conductivity after the formation.Such a structure results in a reduction in manufacturing costs.

The conductive layers 1190, 1210, and 1220 each have alight-transmitting property; accordingly, the capacitors 1070 and 1080each have a light-transmitting property. Note that the conductive layer1220 may be formed using the same material and in the same process asthe conductive layer 1120.

In the case where a light curable adhesive is used for the adhesivelayer 400 in FIG. 7C, the use of a light-absorbing or light-reflectingmaterial for a region in contact with the adhesive layer 400 might causeinsufficient adhesion because the adhesive layer 400 cannot beirradiated with a sufficient amount of light. In one embodiment of thepresent invention, however, a light-transmitting material is used forboth of the conductive layers 1190 and 1210 of the capacitor 1070, andthus, light passes through the capacitor 1070. In addition, alight-transmitting material is used for both of the conductive layers1210 and 1220 of the capacitor 1080, and thus, light passes through thecapacitor 1080. As a result, the adhesive layer 400 can be curedsufficiently and the display device can be manufactured at low costs andwith high productivity.

With the above structure, the light curable adhesive can be cured moresurely and the display device can be manufactured at low costs and withhigh productivity. Furthermore, in the case where the capacitor areacannot be increased in terms of circuit configuration, the capacitancecan be increased without an increase in the capacitor area.

Although an example of using a transistor including an oxidesemiconductor is shown in this embodiment, one embodiment of the presentinvention is not limited to this example. Depending on the case orcircumstances, a transistor including a semiconductor material that isnot an oxide semiconductor may be used in one embodiment or the presentinvention.

For example, a transistor in which a Group 14 element, a compoundsemiconductor, an oxide semiconductor, or the like is used for thesemiconductor layer can be used. Specifically, a semiconductorcontaining silicon, a semiconductor containing gallium arsenide, anorganic semiconductor, or the like can be used.

For example, single crystal silicon, polysilicon, or amorphous siliconcan be used for the semiconductor layer of the transistor.

Note that the electrode of the capacitor is formed using alight-transmitting conductive layer; however, one embodiment of thepresent invention is not limited to this and a metal material or thelike can also be used.

Although the capacitor 1070 in the circuit 30 is shown as an example,the above structure can be used for the capacitor 70 in the circuit 20.As a result, the capacitor area can be reduced, resulting in a reducedpixel area, and further an increased pixel density, so that ahigh-definition display device can be manufactured. The above structurecan also be used for the capacitor in the circuit 40.

Note that this embodiment can be combined with any of the otherembodiments in this specification as appropriate.

Embodiment 4

In this embodiment, the display devices shown in Embodiments 1 to 3 willbe described in detail with reference to FIGS. 8A to 8D.

FIGS. 8A to 8K are examples of top views and a cross-sectional view ofthe display device 10. Note that FIG. 8A illustrates part of a typicalstructure including the circuit 20 (a display region), the circuit 30 (agate driver circuit), the circuit 40 (a source driver circuit), and aflexible printed circuit (FPC) 900. FIG. 8B shows an example of a topview of a pixel circuit included in the circuit 20. FIG. 8C shows anexample of a top view of part of a circuit included in the circuit 30.

FIG. 8D shows a cross-sectional view along a dashed-dotted line C-C′ inFIG. 8A, a cross-sectional view along a dashed-dotted line B-B′ in FIG.8B, and a cross-sectional view along a dashed-dotted line A-A′ in FIG.8C. In a region where the circuit 20 is provided, the substrate 100, thedisplay element (e.g., the liquid crystal element 330 including theliquid crystal layer 200), and the substrate 300 are stacked in thisorder. In a region where the circuit 30 is provided, the substrates 100and 300 are attached to each other with the adhesive layer 400. FIG. 8Dillustrates an example in which a touch sensor 500 overlaps with thesubstrate 300; however, the touch sensor 500 is not necessarilyprovided.

<<Substrates 100 and 300>>

There is no particular limitation on a material and the like of thesubstrates 100 and 300 as long as the material has heat resistance highenough to withstand at least heat treatment performed later. A materialwith a high light-transmitting property is preferably used.

For the substrate 100, an organic material, an inorganic material, acomposite material of an organic material and an inorganic material, orthe like can be used. For example, an inorganic material such as glass,a ceramic, or a metal can be used for the substrate 100.

Specifically, non-alkali glass, soda-lime glass, potash glass, crystalglass, or the like can be used for the substrate 100. An inorganic oxidefilm, an inorganic nitride film, an inorganic oxynitride film, or thelike can be used for the substrate 100. Silicon oxide, silicon nitride,silicon oxynitride, alumina, stainless steel, aluminum, or the like canbe used for the substrate 100.

For example, an organic material such as a resin, a resin film, orplastic can be used for the substrate 100. Specifically, a resin film orresin plate of polyester, polyolefin, polyamide, polyimide,polycarbonate, an acrylic resin, or the like can be used for thesubstrate 100.

For example, a composite material such as a resin film to which a metalplate, a thin glass plate, or a film of an inorganic material isattached can be used for the substrate 100. For example, a compositematerial formed by dispersing a fibrous or particulate metal, glass,inorganic material, or the like into a resin film can be used for thesubstrate 100. For example, a composite material formed by dispersing afibrous or particulate resin, organic material, or the like into aninorganic material can be used for the substrate 100.

Furthermore, a single-layer material or a stacked-layer material inwhich a plurality of layers are stacked can be used for the substrate100. For example, a stacked-layer material in which a base, aninsulating film that prevents diffusion of impurities contained in thebase, and the like are stacked can be used for the substrate 100.Specifically, a stacked-layer material in which glass and one or aplurality of films that prevent diffusion of impurities contained in theglass and that are selected from a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, and the like are stacked canbe used for the substrate 100. Alternatively, a stacked-layer materialin which a resin and a film for preventing diffusion of impurities thatpenetrate the resin, such as a silicon oxide film, a silicon nitridefilm, and a silicon oxynitride film are stacked can be used for thesubstrate 100.

The above-described substrate that can be used as the substrate 100 canbe used as the substrate 300 as well.

<<Insulating Layers 110 and 1100>>

Note that the insulating layers 110 and 1100 having a function as a basefilm are formed using silicon oxide, silicon oxynitride, siliconnitride, silicon nitride oxide, gallium oxide, hafnium oxide, yttriumoxide, aluminum oxide, aluminum oxynitride, or the like. Note that whensilicon nitride, gallium oxide, hafnium oxide, yttrium oxide, aluminumoxide, or the like is used as a material for the insulating layer 110,it is possible to suppress diffusion of impurities such as alkali metal,water, and hydrogen into the oxide semiconductor layer 240 from thesubstrate 100. The insulating layer 110 is formed over the substrate100. The insulating layer 110 is not necessarily provided. Theinsulating layer 1110 is formed using a film having the same compositionas the insulating layer 110.

<<Conductive Layers 120 and 1120>>

The conductive layers 120 and 1120 having a function as a gate electrodeare formed using a metal element selected from aluminum, chromium,copper, tantalum, titanium, molybdenum, nickel, iron, cobalt, andtungsten; an alloy containing any of these metal elements as acomponent; an alloy containing any of these metal elements incombination; or the like. Further, one or more metal elements selectedfrom manganese and zirconium may be used. The conductive layer 120 mayhave a single-layer structure or a layered structure of two or morelayers. For example, any of the following can be used: a single-layerstructure of an aluminum film containing silicon; a single-layerstructure of a copper film containing manganese; a two-layer structurein which a titanium film is stacked over an aluminum film; a two-layerstructure in which a titanium film is stacked over a titanium nitridefilm; a two-layer structure in which a tungsten film is stacked over atitanium nitride film; a two-layer structure in which a tungsten film isstacked over a tantalum nitride film or a tungsten nitride film; atwo-layer structure in which a copper film is stacked over a copper filmcontaining manganese; a three-layer structure in which a titanium film,an aluminum film, and a titanium film are stacked in this order; athree-layer structure in which a copper film containing manganese, acopper film, and a copper film containing manganese are stacked in thisorder; and the like. Alternatively, an alloy film or a nitride filmwhich contains aluminum and one or more elements selected from titanium,tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may beused. The conductive layer 1120 can be formed using a film having thesame composition as the conductive layer 120.

<<Insulating Layers 130 and 1130>>

The insulating layers 130 and 1130 have a function as a gate insulatingfilm. The insulating layer 130 can be formed using. For example, aninsulating film containing at least one of aluminum oxide, magnesiumoxide, silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Theinsulating layer 130 may be a stack of any of the above materials. Theinsulating layer 130 may contain lanthanum (La), nitrogen, or zirconium(Zr) as an impurity. The insulating layer 1130 is formed at the sametime as the insulating layer 130 and is formed using a film having thesame composition as the insulating layer 130.

<<Oxide Semiconductor Layers 240 and 1240>>

The oxide semiconductor layers 240 and 1240 are formed using a metaloxide containing at least In or Zn; typically, an In—Ga oxide, an In—Znoxide, or In-M-Zn oxide (M is Al, Ga, Y, Zr, La, Ce, or Nd), or the likeis used. Note that the oxide semiconductor layer 1240 is formed at thesame time as the oxide semiconductor layer 240 and is formed using afilm having the same composition as the oxide semiconductor layer 240.

When the oxide semiconductor layers 240 and 1240 are formed using anIn-M-Zn oxide, the atomic ratio of In to M when the summation of In andM is assumed to be 100 atomic % is preferably as follows: the proportionof In is higher than 25 atomic % and the proportion of M is lower than75 atomic %; further preferably, the proportion of In is higher than 34atomic % and the proportion of f is lower than 66 atomic %.

The energy gap of each of the oxide semiconductor layers 240 and 1240 is2 eV or more, preferably 2.5 eV or more, and further preferably 3 eV ormore. With the use of an oxide semiconductor having such a wide energygap, the off-state current of the transistor 60 can be reduced.

The thickness of each of the oxide semiconductor layers 240 and 1240ranges from 3 nm to 200 nm, preferably from 3 nm to 100 nm, and furtherpreferably from 3 nm to 50 nm.

In the case where the oxide semiconductor layers 240 and 1240 are formedusing an In-M-Zn oxide (M is Al, Ga, Y, Zr, La, Ce, or Nd), it ispreferable that the atomic ratio of metal elements of a sputteringtarget used for forming the In-M-Zn oxide satisfy In≧M and Zn≧M. As theatomic ratio of metal elements of such a sputtering target,In:M:Zn—1:1:1, In:M:Zn—1:1:1.2, In:M:Zn—3:1:2, and In:M:Zn—4:1:4.1 arepreferable. Note that the atomic ratio of metal elements in the formedoxide semiconductor layers 240 and 1240 varies from the above atomicratio of metal elements of the sputtering target within a range of +40%as an error. Note that a c-axis aligned crystalline oxide semiconductor(CAAC-OS) film and a microcrystalline oxide semiconductor film that aredescribed later can be formed using a target including an In—Ga—Znoxide, preferably a polycrystalline target including an In—Ga—Zn oxide.

Hydrogen contained in the oxide semiconductor reacts with oxygen bondedto a metal atom to be water, and also causes oxygen vacancies in alattice from which oxygen is released (or a portion from which oxygen isreleased). Due to entry of hydrogen into the oxygen vacancies, anelectron serving as a carrier is generated. Further, in some cases,bonding of part of hydrogen to oxygen bonded to a metal atom causesgeneration of an electron serving as a carrier. Thus, a transistorincluding an oxide semiconductor which contains hydrogen is likely to benormally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible as well as the oxygen vacancies in the oxide semiconductorlayers 240 and 1240. Specifically, in the oxide semiconductor layers 240and 1240, the concentration of hydrogen which is measured by secondaryion mass spectrometry (SIMS) is set to lower than or equal to 5×10¹⁹atoms/cm³, preferably lower than or equal to 1×10¹⁹ atoms/cm³, furtherpreferably lower than or equal to 5×10¹⁸ atoms/cm³, still furtherpreferably lower than or equal to 1×10¹⁸ atoms/cm³, yet still furtherpreferably lower than or equal to 5×10¹⁷ atoms/cm³, and still morepreferably lower than or equal to 1×10¹⁶ atoms/cm³. As a result, thetransistor 60 has a positive threshold voltage (also referred to asnormally-off characteristics).

When silicon or carbon which is one of the elements belonging to Group14 is contained in the oxide semiconductor layers 240 and 1240, oxygenvacancies are increased in the oxide semiconductor layers 240 and 1240,and the oxide semiconductor layers 240 and 1240 have n-typeconductivity. Thus, the concentration of silicon or carbon (theconcentration is measured by SIMS) in the oxide semiconductor layers 240and 1240 is lower than or equal to 2×10¹⁸ atoms/cm³, preferably lowerthan or equal to 2×10¹⁷ atoms/cm³. As a result, the transistor 60 has apositive threshold voltage (also referred to as normally-offcharacteristics).

Further, the concentration of alkali metal or alkaline earth metal inthe oxide semiconductor layers 240 and 1240, which is measured by SIMS,is lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than orequal to 2×10⁶ atoms/cm³. Alkali metal and alkaline earth metal mightgenerate carriers when bonded to an oxide semiconductor, in which casethe off-state current of the transistor might be increased. Therefore,it is preferable to reduce the concentration of alkali metal or alkalineearth metal in the oxide semiconductor layers 240 and 1240. As a result,the transistor 60 has a positive threshold voltage (also referred to asnormally-off characteristics).

Further, when nitrogen is contained in the oxide semiconductor layers240 and 1240, electrons serving as carriers are generated to increasethe carrier density, so that the oxide semiconductor layers 240 and 1240easily have n-type conductivity. Thus, the transistor tends to havenormally-on characteristics. For this reason, nitrogen in the oxidesemiconductor layers 240 and 1240 is preferably reduced as much aspossible; for example, the concentration of nitrogen which is measuredby SIMS is preferably set to lower than or equal to 5×10¹⁸ atoms/cm³.

When impurities in the oxide semiconductor layers 240 and 1240 arereduced, the carrier density of the oxide semiconductor layers 240 and1240 can be lowered. Each of the oxide semiconductor layers 240 and 1240has a carrier density of 1×10¹⁵/cm³ or less, preferably 1×10¹³/cm³ orless, further preferably 8×10¹¹/cm³ or less, still further preferably1×10¹¹/cm³ or less, and yet still further preferably 1×10¹⁰/cm³ or lessand 1×10⁹/cm³ or more.

Note that in the oxide semiconductor layers 240 and 1240, a plurality ofoxide semiconductor films that differ in the atomic ratio of metalelements may be stacked. For example, as illustrated in FIG. 9A, theoxide semiconductor layer 240 and an oxide semiconductor layer 241 maybe stacked in order over the insulating layer 130. Alternatively, asillustrated in FIG. 9B, an oxide semiconductor layer 242, the oxidesemiconductor layer 240, and the oxide semiconductor layer 241 may bestacked in order over the insulating layer 130. The oxide semiconductorlayers 241 and 242 differ from the oxide semiconductor layer 240 in theatomic ratio of metal elements. The oxide semiconductor layer 1240 mayalso have the same structure.

When an oxide semiconductor having a low impurity concentration and alow density of defect states is used for the oxide semiconductor layer240, the transistor can have more excellent electrical characteristics.Here, the state in which impurity concentration is low and the densityof defect states is low (the amount of oxygen vacancies is small) isreferred to as “highly purified intrinsic” or “substantially highlypurified intrinsic”. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor has few carrier generationsources, and thus has a low carrier density in some cases. Thus, thetransistor whose channel region is formed in the oxide semiconductorlayer 240 including the oxide semiconductor is likely to have a positivethreshold voltage (also referred to as normally-off characteristics). Ahighly purified intrinsic or substantially highly purified intrinsicoxide semiconductor has a low density of defect states and accordinglyhas a low density of trap states in some cases. The transistor includingthe oxide semiconductor layer 240 having the highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor has anextremely low off-state current; the off-state current can be less thanor equal to the measurement limit of a semiconductor parameter analyzer,i.e., less than or equal to 1×10¹³ A, at a voltage (drain voltage)between a source electrode and a drain electrode of from 1 V to 10 V.Thus, the transistor whose channel region is formed in the oxidesemiconductor layer 240 has a small variation in electricalcharacteristics and high reliability in some cases.

Since the oxide semiconductor layer 240 can be deposited by sputtering,the transistor using the oxide semiconductor can also be used in alarge-area display device.

Note that instead of the oxide semiconductor layer 240, a semiconductorlayer including silicon or silicon germanium may be formed. Thesemiconductor layer including silicon or silicon germanium can have anamorphous structure, a polycrystalline structure, or a single crystalstructure, as appropriate.

<<Conductive Layers 150, 160, 1150, 1160, and 2150>>

The pair of conductive layers 150 and 160 have a function as a sourceelectrode and a drain electrode. The pair of conductive layers 150 and160, the pair of conductive layers 1150 and 1160, and the conductivelayer 2150 are formed using a metal element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, nickel, iron, cobalt,and tungsten; an alloy containing any of these metal elements as acomponent; an alloy containing any of these metal elements incombination; or the like. Further, one or more metal elements selectedfrom manganese and zirconium may be used. The conductive layers 150,160, 1150, 1160, and 2150 may each have a single-layer structure or alayered structure of two or more layers. For example, any of thefollowing can be used: a single-layer structure of an aluminum filmcontaining silicon; a single-layer structure of a copper film containingmanganese; a two-layer structure in which a titanium film is stackedover an aluminum film; a two-layer structure in which a titanium film isstacked over a titanium nitride film; a two-layer structure in which atungsten film is stacked over a titanium nitride film; a two-layerstructure in which a tungsten film is stacked over a tantalum nitridefilm or a tungsten nitride film; a two-layer structure in which a copperfilm is stacked over a copper film containing manganese; a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order; a three-layer structure in which acopper film containing manganese, a copper film, and a copper filmcontaining manganese are stacked in this order; and the like.Alternatively, an alloy film or a nitride film which contains aluminumand one or more elements selected from titanium, tantalum, tungsten,molybdenum, chromium, neodymium, and scandium may be used. The pair ofconductive layers 1150 and 1160, and the conductive layer 2150 areformed using a film having the same composition as the conductive layers150 and 160.

<<Insulating Layers 170, 1170, and 2170>>

The insulating layer 170 has a function of protecting the channel regionof the transistor. The insulating layer 170 is formed using an oxideinsulating film such as silicon oxide, silicon oxynitride, aluminumoxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttriumoxide, yttrium oxynitride, hafnium oxide, or hafnium oxynitride, or anitride insulating film such as silicon nitride or aluminum nitride. Theinsulating layer 170 can have a single-layer structure or astacked-layer structure. The insulating layers 1170 and 2170 are formedusing a film having the same composition as the insulating layer 170.

The insulating layer 170 is preferably formed using an oxide insulatingfilm containing more oxygen than that in the stoichiometric composition.Part of oxygen is released by heating from the oxide insulating filmcontaining more oxygen than that in the stoichiometric composition. Theoxide insulating film containing more oxygen than that in thestoichiometric composition is an oxide insulating film of which theamount of released oxygen atoms is greater than or equal to 1.0×10¹³atoms/cm³, preferably greater than or equal to 3.0×10²⁰ atoms/cm³ inthermal desorption spectroscopy (TDS) analysis in which heat treatmentis performed such that a temperature of a film surface is higher than orequal to 100° C. and lower than or equal to 700° C. or higher than orequal to 100° C. and lower than or equal to 500° C. By the heattreatment, oxygen contained in the insulating layer 170 can betransferred to the oxide semiconductor layer 240, so that the amount ofoxygen vacancies in the oxide semiconductor layer 240 can be reduced.

<<Insulating Layers 180, 1180, and 2180>>

When an insulating film having a blocking effect against oxygen,hydrogen, water, and the like is provided as the insulating layer 180,it is possible to prevent outward diffusion of oxygen from the oxidesemiconductor layer 240 and entry of hydrogen, water, or the like intothe oxide semiconductor layer 240 from the outside. The insulating layer180 can be formed using, for example, an insulating film containing atleast one of aluminum oxide, magnesium oxide, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, and tantalum oxide. The insulating layer180 may be a stack of any of the above materials. The insulating layer180 may contain lanthanum (La), nitrogen, or zirconium (Zr) as animpurity. The insulating layers 1180 and 2180 are formed using a filmhaving the same composition as the insulating layer 180.

<<Conductive Layers 190, 1190, and 2190>>

The conductive layer 190 is formed using a conductive film thattransmits visible light. For example, a material including one of indium(In), zinc (Zn), and tin (Sn) can be used for the conductive film thattransmits visible light. Typically, a conductive oxide such as indiumtin oxide, indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide containing silicon oxide can be used. The conductive layers1190 and 2190 are formed using a film having the same composition as theconductive layer 190.

<<Conductive Layers 250 and 1250>>

The conductive layer 250 as well as the oxide semiconductor layer 240 isformed over the insulating layer 130. The conductive layer 1250 as wellas the oxide semiconductor layer 1240 is formed over the insulatinglayer 1130. The conductive layers 250 and 1250 can be formed using thesame material at the same time.

When the transistor 60 includes the oxide semiconductor layer 240, theconductive layer 250 can be formed over the insulating layer 130 usingthe same material as the semiconductor layer 240. In that case, theconductive layer 250 is formed by processing a film formed at the sametime as the oxide semiconductor layer 240, and therefore containselements similar to those in the oxide semiconductor layer 240. Theconductive layer 250 has a crystal structure similar to or differentfrom that of the oxide semiconductor layer 240. When the film formed atthe same time as the oxide semiconductor layer 240 includes impuritiesor oxygen vacancies, the film can have conductivity to be the conductivelayer 250. Typical examples of the impurities included in the conductivelayer 250 are a rare gas, hydrogen, boron, nitrogen, fluorine, aluminum,and phosphorus. Typical examples of the rare gas include helium, neon,argon, krypton, and xenon. Note that the conductive layer 250 hasconductivity as an example; however, one embodiment of the presentinvention is not limited to this example and the conductive layer 250does not necessarily have conductivity depending on the case orcircumstances. In other words, the conductive layer 250 may haveproperties similar to those of the oxide semiconductor layer 240.

Although the oxide semiconductor layer 240 and the conductive layer 250are formed over the insulating layer 130 in the above manner, they havedifferent impurity concentrations. Specifically, the impurityconcentration of the conductive layer 250 is higher than that of theoxide semiconductor layer 240. For example, in the oxide semiconductorlayer 240, the hydrogen concentration measured by secondary ion massspectrometry is lower than or equal to 5×10¹⁹ atoms/cm³, preferablylower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower thanor equal to 1×10¹⁸ atoms/cm³, still further preferably lower than orequal to 5×10¹⁷ atoms/cm³, and yet still further preferably lower thanor equal to 1×10¹⁶ atoms/cm³. In contrast, the hydrogen concentration inthe conductive layer 250 measured by secondary ion mass spectrometry ishigher than or equal to 8×10¹⁹ atoms/cm³, preferably higher than orequal to 1×10²⁰ atoms/cm³, and further preferably higher than or equalto 5×10²⁰ atoms/cm³. In addition, the hydrogen concentration in theconductive layer 250 is greater than or equal to 2 times or greater thanor equal to 10 times that in the oxide semiconductor layer 240.

The conductive layer 250 has lower resistivity than the oxidesemiconductor layer 240. The resistivity of the conductive layer 250 ispreferably greater than or equal to 1×10⁻⁸ times and less than 1×10⁻¹times that of the oxide semiconductor layer 240. The resistivity of theconductive layer 250 is typically greater than or equal to 1×10⁻³ Ωcmand less than 1×10⁴ Ωcm, or greater than or equal to 1×10⁻³ Ωcm and lessthan 1×10⁻¹ Ωcm.

When an oxide semiconductor film formed at the same time as the oxidesemiconductor layer 240 is exposed to plasma, the oxide semiconductorfilm is damaged and oxygen vacancies can be generated. For example, whena film is formed over the oxide semiconductor film by a plasma CVDmethod or a sputtering method, the oxide semiconductor film is exposedto plasma and oxygen vacancies are generated. Alternatively, when theoxide semiconductor film is exposed to plasma in etching treatment forformation of an opening in the insulating layer 170, oxygen vacanciesare generated. Alternatively, when the oxide semiconductor film isexposed to plasma of a mixed gas of oxygen and hydrogen, hydrogen, arare gas, ammonia, and the like, oxygen vacancies are generated.Alternatively, when impurities are added to the oxide semiconductorfilm, oxygen vacancies can be formed while the impurities are added tothe oxide semiconductor film. The impurities can be added by an iondoping method, an ion implantation method, a plasma treatment method,and the like. In the plasma treatment method, plasma is generated in agas atmosphere containing the impurities to be added, and ions of theimpurities accelerated by plasma treatment are made to collide with theoxide semiconductor film, whereby oxygen vacancies can be formed in theoxide semiconductor film.

When an impurity. e.g., hydrogen is contained in the oxide semiconductorfilm in which oxygen vacancies are generated by addition of impurityelements, hydrogen enters an oxygen vacant site and forms a donor levelin the vicinity of the conduction band. As a result, the oxidesemiconductor film has increased conductivity to be a conductor. Anoxide semiconductor film that has become a conductor can be referred toas an oxide conductor film. That is, it can be said that the oxidesemiconductor layer 240 is formed of an oxide semiconductor and theconductive layer 250 is formed of an oxide conductor film. It can alsobe said that the conductive layer 250 is formed of an oxidesemiconductor film having high conductivity or a metal oxide film havinghigh conductivity.

Note that the insulating layer 180 preferably contains hydrogen. Sincethe conductive layer 250 is in contact with the insulating layer 170,hydrogen contained in the insulating layer 180 can be diffused into theoxide semiconductor film formed at the same time as the oxidesemiconductor layer 240. As a result, impurities can be added to theoxide semiconductor film formed at the same time as the oxidesemiconductor layer 240.

Furthermore, the insulating layer 170 is preferably formed using anoxide insulating film containing more oxygen than that in thestoichiometric composition, and the insulating layer 180 is preferablyformed using an insulating film containing hydrogen. When oxygencontained in the insulating layer 170 is transferred to the oxidesemiconductor layer 240 of the transistor 60, the amount of oxygenvacancies in the oxide semiconductor layer 240 can be reduced and achange in the electrical characteristics of the transistor 60 can bereduced. In addition, hydrogen contained in the insulating layer 180 istransferred to the conductive layer 250 to increase the conductivity ofthe conductive layer 250.

In the above manner, the conductive layer 250 can be formed at the sametime as the oxide semiconductor layer 240 and is given conductivityafter the formation. Such a structure results in a reduction inmanufacturing costs.

The conductive layer 250 is formed at the same time as the oxidesemiconductor layer 240 and is given conductivity after the formation.Such a structure results in a reduction in manufacturing costs.

Conductivity can be given to the conductive layers 250 and 1250 in thesame way.

<<Insulating Layer 600>>

An insulating layer 600 has a function as a planarization film. Theinsulating layer 600 is formed using a heat-resistant organic material,such as a polyimide resin, an acrylic resin, a polyimide amide resin, abenzocyclobutene resin, a polyamide resin, or an epoxy resin. Note thatthe insulating layer 600 may be formed by stacking a plurality ofinsulating films formed using any of these materials. The insulatinglayer 600 is not necessarily provided.

<<Light-Blocking Layer 630>>

A light-blocking material can be used for the light-blocking layer 630.A resin in which a pigment is dispersed, a resin containing a dye, or aninorganic film such as a black chromium film can be used for thelight-blocking layer 630. Carbon black, an inorganic oxide, a compositeoxide containing a solid solution of a plurality of inorganic oxides, orthe like can be used for the light-blocking layer 630.

<<Coloring Layer 650>>

The coloring layer 650 transmits light in a specific wavelength range.For example, a color filter that transmits light in a specificwavelength range, such as red, green, blue, or yellow light, can beused. Each coloring layer is formed in a desired position with any ofvarious materials by a printing method, an inkjet method, an etchingmethod using a photolithography method, or the like. In a white pixel, aresin such as a transparent resin or a white resin may be overlappedwith the light-emitting element.

<<Spacer 350>>

An insulating material can be used for a spacer 350. For example, aninorganic material, an organic material, or a stacked-layer material ofan inorganic material and an organic material can be used. Specifically,a film containing silicon oxide, silicon nitride, or the like, acrylic,polyimide, a photosensitive resin, or the like can be used.

<<FPC 900>>

An FPC 900 is electrically connected to the conductive layer 2190 withan anisotropic conductive film 910 provided therebetween. The conductivelayer 2190 can be formed in the step of forming the electrode layer ofthe transistor 1060 and the like. The FPC 900 can supply an image signaland the like to the circuit 30 (driver circuit) including the transistor1060, the capacitor 1070, and the like.

<Modification Example of Transistor 1060>

A modification example of the transistor 1060 will be described withreference to FIGS. 10A to 10C. The transistor illustrated in FIGS. 10Ato 10C has a dual-gate structure.

FIGS. 10A to 10C are a top view and cross-sectional views of thetransistor 1060 included in a semiconductor device. FIG. 10A is a topview of the transistor 1060, FIG. 10B is a cross-sectional view along adashed-dotted line B-B′ of FIG. 10A, and FIG. 10C is a cross-sectionalview along a dashed-dotted line C-C′ of FIG. 10A. Note that in FIG. 10A,the substrate 100, the insulating layer 110, the insulating layer 1130,the insulating layer 1170, the insulating layer 1180, the adhesive layer400, and the like are not illustrated for the sake of clarity.

The transistor 1060 illustrated in FIGS. 10A to 10C includes theconductive layer 1120 that is over the insulating layer 110 and has afunction as a gate electrode, the insulating layer 1130 that is over theconductive layer 1120 and has a function as a gate insulating film, theoxide semiconductor layer 1240 that overlaps with the conductive layer1120 with the insulating layer 1130 provided therebetween, the pair ofconductive layers 1150 and 1160 in contact with the oxide semiconductorlayer 1240, the insulating layer 1170 over the oxide semiconductor layer1240 and the pair of conductive layers 1150 and 1160, the insulatinglayer 1180 over the insulating layer 1170, and a conductive layer 1230that is over the insulating layer 1180 and has a function as a back-gateelectrode. The conductive layer 1120 may be connected to the conductivelayer 1230 through an opening 1260 formed in the insulating layers 1130,1170, and 1180.

<<Conductive Layer 1230>>

The conductive layer 1230 is formed using a conductive film thattransmits visible light or a conductive film that reflects visiblelight. For example, a material including one of indium (In), zinc (Zn),and tin (Sn) can be used for the conductive film that transmits visiblelight. Typically, a conductive oxide such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxidecontaining silicon oxide can be used. For the conductive film thatreflects visible light, a material containing aluminum or silver can beused, for example.

Note that when a side surface of the oxide semiconductor layer 1240faces the conductive layer 1230 in the channel width direction as shownin FIG. 10C, carriers flow not only at the interface between theinsulating layer 1170 and the semiconductor layer 1140 and at theinterface between the insulating layer 1130 and the semiconductor layer1140 but also in the semiconductor layer 1140. Therefore, the amount oftransfer of carriers in the transistor 1060 is increased. As a result,the on-state current and field-effect mobility of the transistor 1060are increased. The electric field of the conductive layer 1230 affectsthe side surface or an end portion including the side surface and itsvicinity of the semiconductor layer 1140; thus, generation of aparasitic channel at the side surface or the end portion of thesemiconductor layer 1140 can be suppressed.

The transistor illustrated in FIGS. 10A to 10C has higher field-effectmobility and higher on-state current than the transistor illustrated inFIGS. 2A and 2W. Therefore, by using the transistor with the structureillustrated in FIGS. 10A to 10C as the transistor provided in thecircuit 30 (gate driver), a driver circuit portion capable of high-speedoperation can be obtained. Furthermore, the area occupied by the circuit30 can be reduced, and the area of the circuit 20 (pixel portion) can beincreased.

The transistor illustrated in FIGS. 10A to 10C can also be used as thetransistor 60 in the circuit 20. By providing the transistor having ahigh on-state current in the circuit 20 (pixel portion), signal delay inwirings can be reduced and display defects such as display unevennesscan be suppressed even though the number of wirings is increased in alarge-sized display device or a high-resolution display device.

Note that all of transistors 1060 included in the circuit (gate driverand the like) may have the same structure or may have two or more kindsof structures. All of a plurality of transistors 60 included in thecircuit (pixel portion) may have the same structure, or may have two ormore kinds of structures.

Although an example of using a transistor including an oxidesemiconductor is shown in this embodiment, one embodiment of the presentinvention is not limited to this example. Depending on the case orcircumstances, a transistor including a semiconductor material that isnot an oxide semiconductor may be used in one embodiment of the presentinvention.

For example, a transistor in which a Group 14 element, a compoundsemiconductor, an oxide semiconductor, or the like is used for thesemiconductor layer can be used. Specifically, a semiconductorcontaining silicon, a semiconductor containing gallium arsenide, anorganic semiconductor, or the like can be used.

For example, single crystal silicon, polysilicon, or amorphous siliconcan be used for the semiconductor layer of the transistor.

Note that this embodiment can be combined with any of the otherembodiments in this specification as appropriate.

Embodiment 5

A structure of the oxide semiconductor film will be described below.

An oxide semiconductor film is classified into a non-single-crystaloxide semiconductor film and a single crystal oxide semiconductor film.Alternatively, an oxide semiconductor is classified into, for example, acrystalline oxide semiconductor and an amorphous oxide semiconductor.

Examples of a non-single-crystal oxide semiconductor include a c-axisaligned crystalline oxide semiconductor (CAAC-OS), a polycrystallineoxide semiconductor, a microcrystalline oxide semiconductor, and anamorphous oxide semiconductor. In addition, examples of a crystallineoxide semiconductor include a single crystal oxide semiconductor, aCAAC-OS, a polycrystalline oxide semiconductor, and a microcrystallineoxide semiconductor.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

With a transmission electron microscope (TEM), a combined analysis image(also referred to as a high-resolution TEM image) of a bright-fieldimage and a diffraction pattern of the CAAC-OS film is observed.Consequently, a plurality of crystal parts are observed clearly.However, in the high-resolution TEM image, a boundary between crystalparts, i.e., a grain boundary is not observed clearly. Thus, in theCAAC-OS film, a reduction in electron mobility due to the grain boundaryis less likely to occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface, metal atoms are arranged in a layered manner in the crystalparts. Each metal atom layer has a morphology that reflects a surfaceover which the CAAC-OS film is formed (also referred to as a formationsurface) or a top surface of the CAAC-OS film, and is provided parallelto the formation surface or the top surface of the CAAC-OS film.

On the other hand, according to the high-resolution planar TEM image ofthe CAAC-OS film observed in a direction substantially perpendicular tothe sample surface, metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

The CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZn₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic order of theoxide semiconductor film by depriving the oxide semiconductor film ofoxygen and causes a decrease in crystallinity. Furthermore, a heavymetal such as iron or nickel, argon, carbon dioxide, or the like has alarge atomic radius (molecular radius), and thus disturbs the atomicorder of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas “highly purified intrinsic” or “substantially highly purifiedintrinsic.” A highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor film has few carrier generation sources,and thus can have low carrier density. Thus, a transistor including theoxide semiconductor film rarely has negative threshold voltage (israrely normally on). The highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has few carriertraps. Accordingly, the transistor including the oxide semiconductorfilm has few variations in electrical characteristics and highreliability. Charge trapped by the carrier traps in the oxidesemiconductor film takes a long time to be released and may behave likefixed charge. Thus, the transistor that includes the oxide semiconductorfilm having high impurity concentration and high density of defectstates has unstable electrical characteristics in some cases.

In a transistor including the CAAC-OS film, changes in electricalcharacteristics of the transistor due to irradiation with visible lightor ultraviolet light are small.

Next, a microcrystalline oxide semiconductor film is described.

A microcrystalline oxide semiconductor film has a region where a crystalpart is observed in a high-resolution TEM image and a region where acrystal part is not clearly observed in a high-resolution TEM image. Inmost cases, a crystal part in the microcrystalline oxide semiconductorfilm is greater than or equal to 1 nm and less than or equal to 100 nm,or greater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a si e greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as ananocrystalline oxide semiconductor (nc-OS) film. In a high-resolutionTEM image for example, a grain boundary cannot be found clearly in thenc-OS film in some cases.

In the nc-OS film, a microscopic region (e.g., a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has periodic atomic order. There is no regularityof crystal orientation between different crystal parts in the nc-OSfilm. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak that shows a crystalplane does not appear. Furthermore, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter larger than the diameterof a crystal part (e.g., larger than or equal to 50 nm). Meanwhile,spots are shown in a nanobeam electron diffraction pattern of the nc-OSfilm obtained by using an electron beam having a probe diameter close toor smaller than the diameter of a crystal part. Furthermore, in ananobeam electron diffraction pattern of the nc-OS film, regions withhigh luminance in a circular (ring) pattern are observed in some cases.Also in a nanobeam electron diffraction pattern of the nc-OS film, aplurality of spots are shown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularitythan an amorphous oxide semiconductor film. Thus, the nc-OS film has alower density of defect states than the amorphous oxide semiconductorfilm. Note that there is no regularity of crystal orientation betweendifferent crystal parts in the nc-OS film; thus, the nc-OS film has ahigher density of defect states than the CAAC-OS film.

Next, an amorphous oxide semiconductor film is described.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystal part. For example, the amorphous oxide semiconductor filmdoes not have a specific state as in quartz.

In a high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is shown in anelectron diffraction pattern of the amorphous oxide semiconductor film.Furthermore, a halo pattern is shown but a spot is not shown in ananobeam electron diffraction pattern of the amorphous oxidesemiconductor film.

Note that an oxide semiconductor film may have a structure havingphysical properties between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(a-like OS) film.

In a high-resolution TEM image of the a-like OS film, a void may beseen. Furthermore, in the high-resolution TEM image, there are a regionwhere a crystal part is clearly observed and a region where a crystalpart is not observed. In the a-like OS film, crystallization by a slightamount of electron beam used for TEM observation occurs and growth ofthe crystal part is found sometimes. In contrast, crystallization by aslight amount of electron beam used for TEM observation is less observedin the nc-OS film having good quality.

Note that the crystal part size in the a-like OS film and the nc-OS filmcan be measured using high-resolution TEM images. For example, anInGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers areincluded between In—O layers. A unit cell of the InGaZnO₄ crystal has astructure in which nine layers of three In—O layers and six Ga—Zn—Olayers are layered in the c-axis direction. Accordingly, the spacingbetween these adjacent layers is equivalent to the lattice spacing onthe (009) plane (also referred to as a d value). The value is calculatedto be 0.29 nm from crystal structure analysis. Thus, each of the latticefringes in which the spacing therebetween is from 0.28 nm to 0.30 nmcorresponds to the a-b plane of the InGaZnO₄ crystal, focusing on thelattice fringes in the high-resolution TEM image.

The density of an oxide semiconductor film might vary depending on itsstructure. For example, if the composition of an oxide semiconductorfilm is determined, the structure of the oxide semiconductor film can beestimated from a comparison between the density of the oxidesemiconductor film and the density of a single-crystal oxidesemiconductor film having the same composition as the oxidesemiconductor film. For example, the density of an a-like OS film ishigher than or equal to 78.6% and lower than 92.3% of that of thesingle-crystal oxide semiconductor film. In addition, for example, thedensity of an nc-OS film or a CAAC-OS film is higher than or equal to92.3% and lower than 100% of that of the single-crystal oxidesemiconductor film. Note that it is difficult to form an oxidesemiconductor film whose density is lower than 78% of that of thesingle-crystal oxide semiconductor film.

Specific examples of the above are described. For example, in the caseof an oxide semiconductor film with an atomic ratio of In:Ga:Zn 1:1:1,the density of single-crystal InGaZnO₄ with a rhombohedral crystalstructure is 6.357 g/cm³. Thus, for example, in the case of the oxidesemiconductor film with an atomic ratio of In:Ga:Zn=1:1:1, the densityof an a-like OS film is higher than or equal to 5.0 g/cm³ and lower than5.9 g/cm³. In addition, for example, in the case of the oxidesemiconductor film with an atomic ratio of In:Ga:Zn=1:1:1, the densityof an nc-OS film or a CAAC-OS film is higher than or equal to 5.9 g/cm³and lower than 6.3 g/cm³.

Note that single crystals with the same composition do not exist in somecases. In such a case, by combining single crystals with differentcompositions at a given proportion, it is possible to calculate thedensity that corresponds to the density of a single crystal with adesired composition. The density of the single crystal with a desiredcomposition may be calculated using weighted average with respect to thecombination ratio of the single crystals with different compositions.Note that it is preferable to combine as few kinds of single crystals aspossible for density calculation.

Note that an oxide semiconductor film may be a stacked film including,for example, two or more films of an amorphous oxide semiconductor film,an a-like OS film, a microcrystalline oxide semiconductor film, and aCAAC-OS film.

Note that this embodiment can be combined with any of the otherembodiments in this specification as appropriate.

Embodiment 6

In this embodiment, a structure example of a display device of oneembodiment of the present invention will be described with reference toFIGS. 11A to 11C.

[Structure Example]

FIG. 11A is a top view of the display device of one embodiment of thepresent invention. FIG. 11B is a circuit diagram illustrating a pixelcircuit that can be used in the case where a liquid crystal element isused in a pixel in the display device of one embodiment of the presentinvention. FIG. 11C is a circuit diagram illustrating a pixel circuitthat can be used in the case where an organic EL element is used in apixel in the display device of one embodiment of the present invention.

The transistor in the pixel portion can be formed in accordance with theabove embodiments. The transistor can be easily formed as an n-channeltransistor, and thus part of a driver circuit that can be formed usingan n-channel transistor is formed over the same substrate as thetransistor of the pixel portion. With the use of any of the transistorsdescribed in the above embodiments for the pixel portion or the drivercircuit in this manner, a highly reliable display device can beprovided.

FIG. 11A illustrates an example of a top view of an active matrixdisplay device. A pixel portion 701, a scan line driver circuit 702, ascan line driver circuit 703, and a signal line driver circuit 704 areformed over a substrate 700 of the display device. In the pixel portion701, a plurality of signal lines extended from the signal line drivercircuit 704 are arranged and a plurality of scan lines extended from thescan line driver circuit 702 and the scan line driver circuit 703 arearranged. Note that pixels which include display elements are providedin a matrix in respective regions where the scan lines and the signallines intersect with each other. The substrate 700 of the display deviceis connected to a timing control circuit (also referred to as acontroller or a controller IC) through a connection portion such as aflexible printed circuit (FPC).

In FIG. 11A, the scan line driver circuit 702, the scan line drivercircuit 703, and the signal line driver circuit 704 are formed over thesubstrate 700 where the pixel portion 701 is formed. Accordingly, thenumber of components which are provided outside, such as a drivercircuit, can be reduced, so that a reduction in cost can be achieved.Furthermore, if the driver circuit is provided outside the substrate700, wirings would need to be extended and the number of wiringconnections would increase. When the driver circuit is provided over thesubstrate 700, the number of wiring connections can be reduced.Consequently, an improvement in reliability or yield can be achieved.

[Liquid Crystal Display Device]

FIG. 11B illustrates an example of a circuit configuration of the pixel.Here, a pixel circuit which is applicable to a pixel of a VA liquidcrystal display device is illustrated as an example.

This pixel circuit can be applied to a structure in which one pixelincludes a plurality of pixel electrode layers. The pixel electrodelayers are connected to different transistors, and the transistors canbe driven with different gate signals. Accordingly, signals applied toindividual pixel electrode layers in a multi-domain pixel can becontrolled independently.

A gate wiring 712 of a transistor 716 and a gate wiring 713 of atransistor 717 are separated so that different gate signals can besupplied thereto. In contrast, a data line 714 is shared by thetransistors 716 and 717. The transistor described in any of the aboveembodiments can be used as appropriate as each of the transistors 716and 717. Thus, a highly reliable liquid crystal display device can beprovided.

A first pixel electrode layer is electrically connected to thetransistor 716 and a second pixel electrode layer is electricallyconnected to the transistor 717. The first pixel electrode layer and thesecond pixel electrode layer are separated. There is no particularlimitation on the shapes of the first pixel electrode layer and thesecond pixel electrode layer. For example, the first pixel electrodelayer may have a V-like shape.

A gate electrode of the transistor 716 is connected to the gate wiring712, and a gate electrode of the transistor 717 is connected to the gatewiring 713. When different gate signals are supplied to the gate wiring712 and the gate wiring 713, operation timings of the transistor 716 andthe transistor 717 can be varied. As a result, alignment of liquidcrystals can be controlled.

Furthermore, a storage capacitor may be formed using a capacitor wiring710, a gate insulating film functioning as a dielectric, and a capacitorelectrode electrically connected to the first pixel electrode layer orthe second pixel electrode layer.

The multi-domain pixel includes a first liquid crystal element 718 and asecond liquid crystal element 719. The first liquid crystal element 718includes the first pixel electrode layer, a counter electrode layer, anda liquid crystal layer therebetween. The second liquid crystal element719 includes the second pixel electrode layer, a counter electrodelayer, and a liquid crystal layer therebetween.

Note that a pixel circuit of the present invention is not limited tothat shown in FIG. 11B. For example, a switch, a resistor, a capacitor,a transistor, a sensor, a logic circuit, or the like may be added to thepixel circuit illustrated in FIG. 11B.

[Organic EL Display Device]

FIG. 11C illustrates another example of a circuit configuration of thepixel. Here, a pixel structure of a display device using an organic ELelement is shown.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodes andholes are injected from the other of the pair of electrodes, into alayer containing a light-emitting organic compound; thus, current flows.The electrons and holes are recombined, and thus, the light-emittingorganic compound is excited. The light-emitting organic compound returnsto a ground state from the excited state, thereby emitting light. Owingto such a mechanism, this light-emitting element is referred to as acurrent-excitation light-emitting element.

FIG. 11C illustrates an applicable example of a pixel circuit. Here, onepixel includes two n-channel transistors. Note that the metal oxide filmof one embodiment of the present invention can be used for a channelformation region of the n-channel transistor. Further, digital timegrayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving will be described.

A pixel 720 includes a switching transistor 721, a driver transistor722, a light-emitting element 724, and a capacitor 723. A gate electrodelayer of the switching transistor 721 is connected to a scan line 726, afirst electrode (one of a source electrode layer and a drain electrodelayer) of the switching transistor 721 is connected to a signal line725, and a second electrode (the other of the source electrode layer andthe drain electrode layer) of the switching transistor 721 is connectedto a gate electrode layer of the driver transistor 722. The gateelectrode layer of the driver transistor 722 is connected to a powersupply line 727 through the capacitor 723, a first electrode of thedriver transistor 722 is connected to the power supply line 727, and asecond electrode of the driver transistor 722 is connected to a firstelectrode (a pixel electrode) of the light-emitting element 724. Asecond electrode of the light-emitting element 724 corresponds to acommon electrode 728. The common electrode 728 is electrically connectedto a common potential line formed over the same substrate as the commonelectrode 728.

As the switching transistor 721 and the driver transistor 722, any ofthe transistors described in other embodiments can be used asappropriate. In this manner, a highly reliable organic EL display devicecan be provided.

The potential of the second electrode (the common electrode 728) of thelight-emitting element 724 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 727. For example, thelow power supply potential can be GND, 0 V. or the like. The high powersupply potential and the low power supply potential are set to be higherthan or equal to the forward threshold voltage of the light-emittingelement 724, and the difference between the potentials is applied to thelight-emitting element 724, whereby current is supplied to thelight-emitting element 724, leading to light emission. The forwardvoltage of the light-emitting element 724 refers to a voltage at which adesired luminance is obtained, and includes at least a forward thresholdvoltage.

Note that the gate capacitance of the driver transistor 722 may be usedas a substitute for the capacitor 723, so that the capacitor 723 can beomitted. The gate capacitance of the driver transistor 722 may be formedbetween the channel formation region and the gate electrode layer.

Next, a signal input to the driver transistor 722 will be described. Inthe case of a voltage-input voltage driving method, a video signal forsufficiently turning on or off the driver transistor 722 is input to thedriver transistor 722. In order for the driver transistor 722 to operatein a linear region, voltage higher than the voltage of the power supplyline 727 is applied to the gate electrode layer of the driver transistor722. Note that voltage higher than or equal to voltage which is the sumof power supply line voltage and the threshold voltage V_(th) of thedriver transistor 722 is applied to the signal line 725.

In the case of performing analog grayscale driving, a voltage higherthan or equal to a voltage which is the sum of the forward voltage ofthe light-emitting element 724 and the threshold voltage V_(th) of thedriver transistor 722 is applied to the gate electrode layer of thedriver transistor 722. A video signal by which the driver transistor 722is operated in a saturation region is input, so that current is suppliedto the light-emitting element 724. In order for the driver transistor722 to operate in a saturation region, the potential of the power supplyline 727 is set higher than the gate potential of the driver transistor722. When an analog video signal is used, it is possible to supplycurrent to the light-emitting element 724 in accordance with the videosignal and perform analog grayscale driving.

Note that the configuration of the pixel circuit of the presentinvention is not limited to that shown in FIG. 11C. For example, aswitch, a resistor, a capacitor, a sensor, a transistor, a logiccircuit, or the like may be added to the pixel circuit illustrated inFIG. 11C.

In the case where the transistor shown in any of the above embodimentsis used for the circuit shown in FIGS. 11A to 11C, the source electrode(the first electrode) is electrically connected to the low potentialside and the drain electrode (the second electrode) is electricallyconnected to the high potential side. Furthermore, the potential of thefirst gate electrode may be controlled by a control circuit or the likeand the potential described above as an example, e.g., a potential lowerthan the potential applied to the source electrode, may be input to thesecond gate electrode through a wiring that is not illustrated.

For example, in this specification and the like, for example, a displayelement, a display device which is a device including a display element,a light-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ a variety of modes or caninclude a variety of elements. The display element, the display device,the light-emitting element, or the light-emitting device includes atleast one of an electroluminescence (EL) element (e.g., an EL elementincluding organic and inorganic materials, an organic EL element, or aninorganic EL element), an LED (e.g., a white LED, a red LED, a greenLED, or a blue LED), a transistor (a transistor that emits lightdepending on current), an electron emitter, a liquid crystal element,electronic ink, an electrophorectic element, a grating light valve(GLV), a plasma display panel (PDP), a display element using microelectro mechanical system (MEMS), a digital micromirror device (DMD), adigital micro shutter (DMS), MIRASOL (registered trademark), aninterferometric modulator display (IMOD) element, a MEMS shutter displayelement, an optical-interference-type MEMS display element, anelectrowetting element, a piezoelectric ceramic display, a displayelement including a carbon nanotube, and the like. Other than the above,a display medium whose contrast, luminance, reflectance, transmittance,or the like is changed by an electrical or magnetic effect may beincluded. Note that examples of a display device including an EL elementinclude an EL display. Examples of a display device including anelectron emitter include a field emission display (FED) and an SED-typeflat panel display (SED: surface-conduction electron-emitter display).Examples of a display device including a liquid crystal element includea liquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). Examples of a display device including electronic ink.Electronic Liquid Powder (registered trademark), or an electrophoreticelement include electronic paper. In the case of a transflective liquidcrystal display or a reflective liquid crystal display, some or all ofpixel electrodes function as reflective electrodes. For example, some orall of pixel electrodes are formed to contain aluminum, silver, or thelike. In such a case, a memory circuit such as an SRAM can be providedunder the reflective electrodes, leading to lower power consumption.Note that in the case of using an LED, graphene or graphite may beprovided under an electrode or a nitride semiconductor of the LED.Graphene or graphite may be a multilayer film in which a plurality oflayers are stacked. As described above, provision of graphene orgraphite enables easy formation of a nitride semiconductor thereover,such as an n-type GaN semiconductor layer including crystals.Furthermore, a p-type GaN semiconductor layer including crystals or thelike can be provided thereover, and thus the LED can be formed. Notethat an AlN layer may be provided between the n-type GaN semiconductorlayer including crystals and graphene or graphite. The GaN semiconductorlayers included in the LED may be formed by MOCVD. Note that when thegraphene is provided, the GaN semiconductor layers included in the LEDcan also be formed by a sputtering method.

Note that this embodiment can be combined with any of the otherembodiments in this specification as appropriate.

Embodiment 7

In this embodiment, examples of an electronic device to which thedisplay device of one embodiment of the present invention can be appliedwill be described with reference to FIGS. 12A to 12D.

Examples of an electronic device including the display device includetelevision sets (also referred to as televisions or televisionreceivers), monitors of computers or the like, cameras such as digitalcameras or digital video cameras, digital photo frames, mobile phones(also referred to as cellular phones or mobile phone devices), portablegame machines, portable information terminals, audio reproducingdevices, and large game machines such as pachinko machines. Specificexamples of these electronic devices are illustrated in FIGS. 12A to12D.

FIG. 12A illustrates a portable game machine including a housing 7101, ahousing 7102, a display portion 7103, a display portion 7104, amicrophone 7105, speakers 7106, an operation key 7107, a stylus 7108,and the like. The display device according to one embodiment of thepresent invention can be used for the display portion 7103 or thedisplay portion 7104. When the display device according to oneembodiment of the present invention is used as the display portion 7103or 7104, it is possible to provide a user-friendly portable game machinewith quality that hardly deteriorates. Although the portable gamemachine illustrated in FIG. 12A includes two display portions, thedisplay portion 7103 and the display portion 7104, the number of displayportions included in the portable game machine is not limited to two.

FIG. 12B illustrates a smart watch, which includes a housing 7302, adisplay portion 7304, operation buttons 7311 and 7312, a connectionterminal 7313, a band 7321, a clasp 7322, and the like. The displaydevice according to one embodiment of the present invention can be usedfor the display portion 7304.

FIG. 12C illustrates a portable information terminal, which includes adisplay portion 7502 incorporated in a housing 7501, operation buttons7503, an external connection port 7504, a speaker 7505, a microphone7506, and the like. The display device of one embodiment of the presentinvention can be used for the display portion 7502.

FIG. 12D illustrates a video camera, which includes a first housing7701, a second housing 7702, a display portion 7703, operation keys7704, a lens 7705, a joint 7706, and the like. The operation keys 7704and the lens 7705 are provided for the first housing 7701, and thedisplay portion 7703 is provided for the second housing 7702. The firsthousing 7701 and the second housing 7702 are connected to each otherwith the joint 7706, and the angle between the first housing 7701 andthe second housing 7702 can be changed with the joint 7706. Imagesdisplayed on the display portion 7703 may be switched in accordance withthe angle at the joint 7706 between the first housing 7701 and thesecond housing 7702. The display device according to one embodiment ofthe present invention can be used for the image display portion 7703.

Note that this embodiment can be combined with any of the otherembodiments in this specification as appropriate.

This application is based on Japanese Patent Application serial No.2014-181552 filed with Japan Patent Office on Sep. 5, 2014, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A display device comprising: a display region over asubstrate and comprising a plurality of pixels; and a gate drivercircuit over the substrate and adjacent to the display region, the gatedriver circuit comprising: a first transistor configured to control anelectrical connection between a clock terminal and an output terminal;and a first capacitor comprising a first electrode connected to a gateof the first transistor and a second electrode connected to the outputterminal, wherein the first electrode and the second electrode each havea light-transmitting property.
 3. The display device according to claim2, wherein the first transistor comprises an oxide semiconductor layercomprising a channel formation region.
 4. The display device accordingto claim 3, wherein the oxide semiconductor layer and any of the firstelectrode and the second electrode comprise a same material.
 5. Thedisplay device according to claim 2, wherein the first electrode isconnected to a source or a drain of the first transistor.
 6. The displaydevice according to claim 2, wherein the second electrode is connectedto a source or a drain of the first transistor.
 7. The display deviceaccording to claim 2, wherein any of the first electrode and the secondelectrode is over and in contact with a gate insulating layer of thefirst transistor.
 8. The display device according to claim 2, whereinthe first transistor and the first capacitor do not overlap with eachother.
 9. The display device according to claim 2, comprising anadhesive layer over the gate driver circuit.
 10. A display devicecomprising: a display region over a substrate and comprising a pluralityof pixels; and a gate driver circuit over the substrate and adjacent tothe display region, wherein the gate driver circuit comprises: a firsttransistor is configured to output a clock signal to the display regionthrough the first transistor and an output terminal; and a firstcapacitor comprising a first electrode connected to a gate of the firsttransistor and a second electrode connected to the output terminal,wherein the first electrode and the second electrode each have alight-transmitting property.
 11. The display device according to claim10, wherein the first transistor comprises an oxide semiconductor layercomprising a channel formation region.
 12. The display device accordingto claim 11, wherein the oxide semiconductor layer and any of the firstelectrode and the second electrode comprise a same material.
 13. Thedisplay device according to claim 10, wherein the first electrode isconnected to a source or a drain of the first transistor.
 14. Thedisplay device according to claim 10, wherein the second electrode isconnected to a source or a drain of the first transistor.
 15. Thedisplay device according to claim 10, wherein any of the first electrodeand the second electrode is over and in contact with a gate insulatinglayer of the first transistor.
 16. The display device according to claim10, wherein the first transistor and the first capacitor do not overlapwith each other.
 17. The display device according to claim 10,comprising an adhesive layer over the gate driver circuit.
 18. A displaydevice comprising: a display region over a substrate and comprising aplurality of pixels, each of the plurality of pixels comprises: a firsttransistor; and a liquid crystal element connected to the firsttransistor; and a gate driver circuit over the substrate and adjacent tothe display region, the gate driver circuit comprising: a secondtransistor configured to control an electrical connection between aclock terminal and an output terminal; and a first capacitor comprisinga first electrode connected to a gate of the second transistor and asecond electrode connected to the output terminal, wherein the firstelectrode and the second electrode each have a light-transmittingproperty.
 19. The display device according to claim 18, wherein thesecond transistor comprises an oxide semiconductor layer comprising achannel formation region.
 20. The display device according to claim 19,wherein the oxide semiconductor layer and any of the first electrode andthe second electrode comprise a same material.
 21. The display deviceaccording to claim 18, wherein the first electrode is connected to asource or a drain of the second transistor.
 22. The display deviceaccording to claim 18, wherein the second electrode is connected to asource or a drain of the second transistor.
 23. The display deviceaccording to claim 18, wherein any of the first electrode and the secondelectrode is over and in contact with a gate insulating layer of thesecond transistor.
 24. The display device according to claim 18, whereinthe second transistor and the first capacitor do not overlap with eachother.
 25. The display device according to claim 18, comprising anadhesive layer over the gate driver circuit.